1LT with stock brakes now.... I got a good deal on SS f&r calipers, front rotors and new pads.... Can I just install the fronts while I wait to get larger rear rotors? Or is it better to wait and do it all at once?

You'll be just fine doing the fronts for now. If doing stainless lines, get the ones for the SS and just keep the rear lines on a shelf until you get the rear calipers. I recommend the Goodridge SS lines; Part # 12219.